Plastic ball grid array with integral heatsink

ABSTRACT

A plastic ball grid array semiconductor package employs a metal heat spreader having supporting arms embedded in the molding cap, in which the embedded supporting arms are not directly affixed to the substrate or in which any supporting arm that is affixed to the substrate is affixed using a resilient material such as an elastomeric adhesive. Also, a process for forming the package includes steps of placing the heat spreader in a mold cavity, placing the substrate over the mold cavity such that the die support surface of the substrate contacts the supporting arms of the heat spreader, and injecting the molding material into the cavity to form the molding cap. The substrate is positioned in register over the mold cavity such that as the molding material hardens to form the mold cap the embedded heat spreader becomes fixed in the appropriate position in relation to the substrate. Also, a process for forming the package includes steps of placing the heat spreader onto the substrate such that at least one of the supporting arms of the heat spreader is affixed to the substrate using a resilient fixative such as an elastomeric adhesive, placing a mold cavity over the heat spreader, and injecting the molding material into the cavity. The elastomeric adhesive holds the heat spreader in the appropriate position in relation to the substrate during injection of the molding material, and as the molding material hardens to form the mold cap the embedded heat spreader becomes fixed in the appropriate position in relation to the substrate. In some embodiments the under surface of the heat spreader at the interface between the heat spreader and the molding compound is roughened, or includes a black copper oxide layer, to improve adhesion and contact between the heat spreader and the molding material. The invention can provide significant improvements in manufacturability and reliability in use.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional of application Ser. No.09/919,763 filed Jul. 31, 2001, which is hereby incorporated byreference in its entirety.

BACKGROUND

[0002] This invention relates to high performance semiconductor devicepackaging.

[0003] Semiconductor devices increasingly require lower cost packagingwith higher thermal and electrical performance. A common package usedfor high performance devices is the Plastic Ball Grid Array (“PBGA”).The PBGA is a surface mount package that can provide higher thermal andelectrical performance, and a lower thickness profile and a smallerfootprint, as compared to leadframe based surface mount packages such asPlastic Quad Flat Package (“PQFP”) and others. Improvements are soughtin the structure and design of the package, to provide increased thermaland electrical performance and to maintain the established footprint andthickness characteristics of standard PBGAs.

[0004] In conventional PBGAs a small fraction of the heat generated bythe semiconductor device dissipates to the ambient through the moldingcompound, principally at the upper surface of the package, and, to amuch lesser extent, through the sides. Most of the heat that isgenerated by the semiconductor device in standard PBGAs is conductedthrough the solder balls to the product board, and the board acts as aheat sink.

[0005] Various approaches have been employed or suggested for increasingpower dissipation from PBGAs. For example, power dissipation to theambient can be increased by blowing air over the package; but costconsiderations or space limitations may make such air cooling approachesimpractical. And, for example, power dissipation can be increased byincreasing the number of solder balls between the package and the board,and, particularly, by increasing the number of balls directly beneaththe device; and by using a laminate substrate having multiple metallayers. These approaches require increases in package dimensions andchanges in the package structure.

[0006] In another approach to increasing power dissipation from PBGAs,often referred to as “Thermally Enhanced PBGA” or TEPBGA, a partiallyembedded metal heat spreader is employed. The partially embedded metalheat spreader includes an upper generally planar portion having a topsurface free of the molding compound and exposed to ambient; andembedded portions, which may be referred to as supporting arms,extending from the upper portion downward to the substrate and attachedat the lower ends to the upper or die support surface of the substrate.

[0007] Conventionally, TEPBGAs having partially embedded heat spreadersare formed generally as follows. A completed ball grid array isprovided, with the die attached on the die support surface of thesubstrate and connected to the substrate, e.g. by wire bonds. A heatspreader is placed on the support surface of the substrate over the die,with the heat spreader supporting arms rigidly affixed onto thesupporting surface using, for example, a cement or adhesive, such as anepoxy cement or adhesive or glue. Then this assembly is placed over amold cavity in an injection mold, so that the upper surface of the upperportion of the heat sink is at the bottom of the mold cavity and thesubstrate surface contacts the edges of the upper opening of the moldcavity. Then the molding compound is injected into the cavity, where itencloses the die and the wire bonds and the supporting arms of the heatspreader and fills the space between the upper surface of the die andthe upper portion of the heat spreader. The molding material hardens toform the mold cap, and the completed assembly is released from the mold.

SUMMARY

[0008] Manufacturing defects can occur in the conventional process forforming a TEPBGA with a partially embedded heat spreader, as a result ofstress on the rigid attachment of the supporting arms to the substratesurface. Particularly, apparently, flexing of the supporting arms duringprocessing can result in breakage of the substrate. Moreover, becausethere is a significant difference in the coefficient of thermalexpansion of the material of the heat spreader, which is a metal such ascopper, and the coefficient of thermal expansion of the substratematerial, stresses develop during temperature cycling when the device isin use, and such stresses can create cracks in the package substrate,leading to package and device failure. We have discovered that suchmanufacturing defects and stress effects can be avoided by eitherperforming the injection molding process without affixing the supportingarms to the substrate at all, or by employing a resilient material suchas an elastomeric adhesive to affix one or more of the supporting armsto the substrate surface prior to injection molding. The resultingpackage is less subject to thermal cycling-relayed stress and stressdamage, because relief is provided between the supporting arms of theheat spreader and the substrate. This is provided according to theinvention because there is either no direct attachment at all betweenthe heat spreader support arms and the substrate or, if there is directattachment of one or more of the support arms to the substrate, it is aresilient joint.

[0009] Accordingly, in one general aspect the invention features amethod for manufacturing a plastic ball grid array package, by placing aheat spreader having an upper portion and a plurality of support armsinto a mold cavity; placing over the mold cavity a ball grid arrayincluding a semiconductor die mounted on a support surface of asubstrate and connected to the substrate, such that lower ends of thesupport arms contact the support surface of the substrate peripheral tothe die; injecting molding material into the cavity to form the moldingcap; and permitting the molding material to harden to form a mold cap.

[0010] In another general aspect the invention features a process forforming a TEPBGA with a partially embedded heat spreader, by placing aheat spreader having an upper portion and a plurality of support armsonto the die support surface of a substrate such that at least one ofthe supporting arms of the heat spreader is affixed to the substrateusing a resilient fixative such as an elastomeric adhesive; placing amold cavity over the heat spreader; injecting the molding material intothe cavity; and permitting the molding material to harden to form themold cap. The resilient fixative holds the heat spreader in theappropriate position in relation to the substrate during injection ofthe molding material, and as the molding material hardens to form themold cap the partially embedded heat spreader becomes fixed in theappropriate position in relation to the substrate.

[0011] In another general aspect the invention features a plastic ballgrid array semiconductor package including a metal heat spreader havingsupporting arms embedded in the molding cap, in which the embeddedsupporting arms are free of direct rigid affixation to the substrate; orin which any supporting arm that is affixed to the substrate is affixedusing a resilient material such as an elastomeric adhesive.

[0012] In some embodiments the heat spreader is constructed of metaland, in particular embodiments, the heat spreader is constructed ofcopper. In some embodiments the heat spreader has four supporting arms,configured so that their lower ends contact the substrate surface in agenerally rectangular, preferably generally square, array. In someembodiments the resilient material has an elastic modulus in the range0.5 MPa to 100 MPa, preferably in the range 1 MPa to 10 MPa, and inparticular embodiments the resilient material has an elastic modulus of5.5 MPa. In some embodiments the resilient material is an elastomericadhesive, for example a silicon adhesive such as the adhesivecommercially available as Dow Corning 7920.

[0013] In some embodiments the elements of the package are selected sothat the overall dimensions of the package are within standardspecifications (and, particularly, so that the overall package thicknessis about the same as or less than that of standard PBGA packages).Particularly, for example, in some embodiments the thicknesses of thedie plus die attach epoxy, the wire bond loop height and thewire-to-mold clearance are determined so that the height from thesubstrate to the top of the package (that is, the sum of the overallmold cap thickness plus the thickness of the heat spreader and thethickness of the heat spreader adhesive) is no more than 1.17 mm. And,for example, in some embodiments the thicknesses of the portions ofelements situated between the semiconductor device and the heatspreader—that is, the elements that lie in the critical thermal path—aredetermined so as to minimize the length of the critical thermal path.

[0014] Particularly, for example, in some embodiments at least part ofthe molding material between the die and the upper portion of the heatspreader is made as thin as is practicable while avoiding contactbetween the upper surface of the die and the under surface of the heatspreader, and the upper portion of the heat spreader is accordinglythickened by downward protrusion of the lower surface. As a result,there is an increased proportion of metal in the path between thesemiconductor device and the upper surface of the package, so that thecombined thermal resistance on the critical heat path from the die tooverlying ambient is reduced. And, for example, in some embodiments atleast some portion of the volume between the die and the upper portionof the heat spreader is occupied by a material having a lower thermalresistivity than the molding material.

[0015] In some particularly preferred embodiments a black copper oxideor a chemically roughened copper under surface of the heat spreader isemployed in order to enhance the adhesion between the bottom of the heatspreader and the mold compound. The under surface of the heat spreaderis treated to provide the black copper oxide layer or chemicallyroughened surface before injection of the molding material. Where ablack oxide is employed, it can be formed by, for example, exposing thecopper surface to NaClO₂ for a time sufficient to form the layer to adesired thickness, preferably in the range about 3 um to 15 um and inparticular embodiments about 7 um thick. Or, where a chemicallyroughened copper surface is employed, it can be formed by amicro-etching process such as a conventional H₂SO₄—H₂O₂ process or otherchemical process, to provide the desired roughness, preferably in therange 0.5 um to 1.0 um and in particular embodiments about 0.5 um.

[0016] The invention can provide excellent power dissipation in apackage more reliably manufacturable than conventional packages, andless likely to fail during thermal cycling in use than conventionalpackages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a diagrammatic sketch in a transverse sectional viewthru a conventional thermally enhanced plastic ball grid array package,having a partially embedded heat spreader with supporting arms rigidlyaffixed to the substrate.

[0018]FIG. 2 is a diagrammatic sketch in a transverse sectional viewthru a portion of an improved thermally enhanced plastic ball grid arraypackage according to an embodiment of the invention.

[0019]FIG. 3 is a diagrammatic sketch in a transverse sectional viewthru a portion of an improved thermally enhanced plastic ball grid arraypackage according to another embodiment of the invention.

[0020]FIG. 4 is a diagrammatic sketch in a transverse sectional viewthru an improved thermally enhanced plastic ball grid array package withan enlarged portion showing the under surface of the heat spreader atthe interface with underlying material according to an embodiment of theinvention.

[0021]FIG. 5 is a diagrammatic sketch in a transverse sectional viewthru an improved thermally enhanced plastic ball grid array packageaccording to still another embodiment of the invention, showing lowerthermal resistivity material interposed between the semiconductor deviceand the overlying portion of the heat spreader.

[0022]FIG. 6 is a diagrammatic sketch in a transverse sectional viewthru an improved thermally enhanced plastic ball grid array packageaccording to still another embodiment of the invention, showing athicker center portion of the heat spreader and correspondingly thinnerportion of the mold cap overlying the semiconductor device.

DETAILED DESCRIPTION

[0023] The invention will now be described in further detail byreference to the drawings, which illustrate alternative embodiments ofthe invention. The drawings are diagrammatic, showing features of theinvention and their relation to other features and structures, and arenot made to scale. For improved clarity of presentation, in the Figs.illustrating embodiments of the invention, elements corresponding toelements shown in other drawings are not all particularly renumbered,although they are all readily identifiable in all the Figs.

[0024] Turning now to FIG. 1, there is shown in a diagrammatic sectionalview a thermally enhanced plastic ball grid array (“TEPBGA”) packagethat is widely used in the industry. This structure makes use of a metalheat spreader 202, partially embedded in the molding cap, with embeddedportions attached to the substrate, and having a circular upper portion206 having an upper surface 209 free of molding compound and exposed tothe ambient. Such a construct can provide power dissipation to as muchas 3.9 Watts with no airflow, and to as much as 4.2 Watts under airflowof 100 lfpm. The improved heat dissipation is a consequence of increasedmetal content of the package and contributions from particularly twodesign factors.

[0025] One design factor that contributes to improved thermalperformance in the PBGA package of FIG. 1 is the reduction of thermalresistance of the path above the device, that is, between the uppersurface of the device and the surface of the package, allowing greaterheat flow to the top and to the ambient. The thermal resistance of thispath is the sum of the thermal resistance of upper portion 206 of theheat spreader adjacent the upper surface 209, having thickness E, andthe thermal resistance of the molding compound 204, having thickness Gbetween the upper surface of the device and the undersurface of theupper portion 206 of the heat spreader. Because the thermal conductivityof the metal of which the heat spreader is formed is typically at least100 times the thermal conductivity of the molding compound, an increasein the proportion of thickness of the metal decreases thermal resistanceand increases heat flow from the device to the top of the package. As apractical matter the maximum thickness E of the upper portion 206 of theheat spreader in this configuration is limited to about 0.30 mm by themold cap thickness A and by the need to accommodate within the thicknessof the mold cap the die and die attach epoxy, which have a combinedthickness B, as well as the wire loops 207, which extend a dimension Dabove the upper surface of the die and which must be kept away fromcontact with the under surface of the upper portion 206 of the heatspreader, by a clearance dimension C. Some heat is conducted to the topby way of the sidewalls 210 of the heat spreader, but this heat path tothe device is longer and less conductive. The following dimensions aretypical for commonly used thermally enhanced PBGA packages of the kindshown in FIG. 2: mold cap thickness A, 1.17 mm; die+die attach epoxythickness B, 0.38 mm; wire bond loop height D, 0.33 mm; heat spreaderthickness E, 0.30 mm; wire loop clearance C, 0.16 mm.

[0026] Another design factor that contributes to improved thermalperformance in the PBGA package of FIG. 1 is the exposed circular heatspreader surface 209 which, with a diameter V in widely-usedconfigurations of 22 mm, which conducts more heat to ambient as comparedwith a surface of molding compound. Heat conduction is generallyproportional to the area of the heat spreader surface 209, but as apractical matter the area is limited usually to about 50% of the uppersurface of the mold cap.

[0027] According to the invention, improved manufacturability andreduced thermal stress failure is provided by eliminating the use of arigid attachment of the supporting arms of the heat spreader duringmanufacture.

[0028]FIG. 2 shows a detail of an embodiment according to the inventionin which the heat spreader 301 is attached to the substrate 304 by aspot 302 of an elastomeric adhesive on the die support surface 310 ofthe substrate at the location where at least one of the supporting arms306 rests. The lower end of the supporting arm may be configured toprovide a “foot”, as shown for example at 308 in FIG. 2. The use of anelastomer adhesive material minimizes the stress due to the coefficientof thermal expansion difference between material of the heat spreader,which is, for example, copper, and which is embedded in the moldcompound 303, and the material of the package substrate 304. Thisembodiment can be made by applying a spot of the uncured adhesive ontothe substrate surface at a location where at least one of the heatspreader supporting arms is to rest, then placing the heat spreader ontothe substrate surface over the die, so that the foot 308 of thesupporting arm (or arms) 306 contacts the spot of adhesive, and thenallowing the adhesive to cure, thereby resiliently fixing the heatspreader in place on the substrate. Or, the spot of adhesive can beplaced on a foot 308 of one or more of the supporting arms 306 and thenbrought into contact with the substrate surface at the appropriate pointand allowed to cure.

[0029]FIG. 3 shows a detail of an embodiment of the invention in whichthe heat spreader 401 is free of direct attachment to the substrate 404.Here, the heat spreader 401 is held in place in the appropriate positionon the support surface 410 of the substrate 404 by the embedding moldcomposition 403, but there is no direct fixative joining the foot 408 ofthe supporting arm 406 to the substrate surface 410 at the resting place402. Here, too, thermal cycling stress between the heat spreader and thesubstrate at the resting place 402 is minimized, because there is norigid connection there. This can increase the reliability of the packagein use.

[0030]FIG. 4 shows at 500 a detail of the interface 509 between theupper portion of the heat spreader 501 and the underlying mold compound502. This is a critical area as this is the key thermal path throughwhich heat generated by the device can escape from the top of the diethrough the mold compound to the heatslug and out of the package topsurface. Any gap or delamination at this interface during board mount,or over time in use, can severely impact the thermal performance of thepackage. Therefore, in some particularly preferred embodiments a blackcopper oxide or a chemically roughened copper under surface of the heatspreader is employed in order to enhance the adhesion between the bottomof the heat spreader and the mold compound. Where a black oxide isemployed, it can be formed by, for example, exposing the copper surfaceto NaClO₂ for a time sufficient to form the layer. In particularembodiments the treatment parameters are designed to produce a blackcopper oxide layer about 7 um thick; preferably the thickness is in therange 3 um to 15 um. Or, where a chemically roughened copper surface isemployed, a micro-etching process can be employed, such as aconventional H₂SO₄—H₂O₂ process or other chemical process, as describedfor example in T. Kida et al. “Improving Dry-Film Adhesion”, July 2001Optoelectronics Manufacturing Conference, published on the internet atwww.pcfab.com, hereby incorporated herein by reference. In particularembodiments the chemical roughening process parameters are designed toproduce a surface roughness about 0.5 um; preferably the roughness is inthe range about 0.5 um to 1.0 um.

[0031]FIG. 5 shows an alternative embodiment of the package according tothe invention in which a thermally conductive material 609 is placedbetween the top 607 of the die and the bottom 608 of the heat spreader602, 610. In this embodiment this thermally conductive material 609 isdefined having a thermal conductivity greater than that of typical moldcompounds (that is, greater than 0.7 W/mK). The material can be a rigidepoxy or, in some embodiments, can be an elastomeric material to providestress relief.

[0032]FIG. 6 shows yet another embodiment according to the invention, inwhich the heat spreader 702, 710 has been modified such that it isthicker in a mid portion 706 to minimize the length of the heat pathbetween the die top 707 and heat spreader bottom 708, yet at the sametime maintaining the desired package profile. Such thickening can beaccomplished through the thickening of the metal in this area of theheat spreader or alternatively, through application of a thermallyconductive material (as defined with reference to FIG. 5) in thisregion. Such a modification can significantly enhance thermalperformance to 20% or more over conventional PBGA.

[0033] Other embodiments are within the following claims.

What is claimed is:
 1. A method for manufacturing a plastic ball gridarray package, comprising placing a heat spreader having an upperportion and a plurality of support arms into a mold cavity; placing overthe mold cavity a ball grid array including a semiconductor die mountedon a support surface of a substrate and connected to the substrate, suchthat lower ends of the support arms contact the support surface of thesubstrate peripheral to the die; injecting molding material into thecavity to form the molding cap; and permitting the molding material toharden to form a mold cap.
 2. The method of claim 1 wherein the heatspreader is made of metal, and further comprising treating anundersurface of the metal heat spreader to form a black copper oxidelayer prior to injecting the molding material.
 3. The method of claim 2wherein the treating comprises exposing a copper undersurface of theheat spreader with NaClO₂ to form a black copper oxide layer.
 4. Themethod of claim 2 wherein the treating comprises exposing the copperundersurface of the heat spreader with NaClO₂ under conditionssufficient to form a black copper oxide layer having a thickness in therange 3 um to 15 um.
 5. The method of claim 4 wherein the treatingcomprises exposing the copper undersurface of the heat spreader withNaClO₂ under conditions sufficient to form a black copper oxide layerhaving a thickness of 7 um.
 6. The method of claim 1 wherein the heatspreader is made of metal, and further comprising treating anundersurface of the metal heat spreader to roughen the undersurfaceprior to injecting the molding material.
 7. The method of claim 6wherein the treating comprises micro-etching the copper undersurface ofthe heat spreader.
 8. The method of claim 7 wherein the treatingcomprises micro-etching the copper undersurface of the heat spreader toa roughness in the range 0.5 um to 1.0 um.
 9. The method of claim 8wherein the treating comprises micro-etching the copper undersurface ofthe heat spreader to a roughness of 0.5 um.
 10. A method formanufacturing a plastic ball grid array package, comprising placing aheat spreader having an upper portion and a plurality of support armsonto the die support surface of a substrate such that at least one ofthe supporting arms of the heat spreader is affixed to the substrateusing a resilient fixative such as an elastomeric adhesive; placing amold cavity over the heat spreader; injecting the molding material intothe cavity; and permitting the molding material to harden to form a moldcap.
 11. The method of claim 10 wherein the heat spreader is made ofmetal, and further comprising treating an undersurface of the metal heatspreader to form a black copper oxide layer prior to placing the heatspreader onto the die support surface of the substrate.
 12. The methodof claim 11 wherein the treating comprises exposing a copperundersurface of the heat spreader with NaClO₂ to form a black copperoxide layer.
 13. The method of claim 11 wherein the treating comprisesexposing the copper undersurface of the heat spreader with NaClO₂ underconditions sufficient to form a black copper oxide layer having athickness in the range 3 um to 15 um.
 14. The method of claim 13 whereinthe treating comprises exposing the copper undersurface of the heatspreader with NaClO₂ under conditions sufficient to form a black copperoxide layer having a thickness of 7 um.
 15. The method of claim 10wherein the heat spreader is made of metal, and further comprisingtreating an undersurface of the metal heat spreader to roughen theundersurface prior to injecting the molding material.
 16. The method ofclaim 15 wherein the treating comprises micro-etching the copperundersurface of the heat spreader.
 17. The method of claim 16 whereinthe treating comprises micro-etching the copper undersurface of the heatspreader to a roughness in the range 0.5 um to 1.0 um.
 18. The method ofclaim 17 wherein the treating comprises micro-etching the copperundersurface of the heat spreader to a roughness of 0.5 um.